Journal of Chemical Physics, Vol.107, No.3, 691-701, 1997
CH-Stretching Overtone Spectra and Internal Methyl Rotation in 2,6-Difluorotoluene
Vapor phase overtone spectra of 2,6-difluorotoluene are recorded in the Delta upsilon(CH)=2 and 3 regions by conventional near-infrared spectroscopy and in the Delta upsilon(CH)=4-6 regions by intracavity dye/titanium : sapphire laser photoacoustic spectroscopy. The spectra are interpreted on the basis of nb initio calculations at the HF/6-31G* level. The methyl regions of the spectra are complex due to coupling between the nearly freely rotating methyl rotor and CH stretching. A model has been developed to predict the methyl spectral profiles, which uses the harmonically coupled anharmonic oscillator local mode model and the rigid rotor model for stretching and torsion, respectively. A dipole moment function is formulated which combines a Taylor series for CH stretching and a Fourier series for torsion. The dipole moment function includes both angular dependence and higher order expansion terms in the CH-stretching coordinate. The model is successful in predicting the methyl overtone spectral profiles and attributes these profiles to a very large number of transitions that arise from terms involving torsion-stretching coupling, both in the Hamiltonian and in the dipole moment function.
Keywords:LOCAL MODE MODEL;VIBRATIONAL OVERTONE;P-FLUOROTOLUENE;PHOTOACOUSTIC-SPECTROSCOPY;SUBSTITUTED TOLUENES;MOLECULAR-STRUCTURE;VAPOR-PHASE;INTENSITIES;DICHLOROMETHANE;XYLENES